Skip to Content

Express Course

How to measure the substrate moisture

Measuring of moisture in the floor substrate is a necessary preparatory step before tiling.

Crucial to know

find out below the crucial values of moisture for different kinds of screed 

Tools needed

CM moisture measuring set

CRH moisture measuring set

Step 1

Moisture in the Subfloor – Measurement Methods

Measuring moisture in the subfloor

Measuring the moisture in the substrate is a must, because it influences the following decisions:

  • Which type of primer is needed?
  • What leveling compound should be applied?
  • Which type of tile or slab can be installed?

There are two methods for on-site moisture measurement:

  1. CM method (calcium carbide method). The moisture content of screeds is measured with the help of a CM moisture meter. 
  2. CRH method (corresponding relative humidity). Moisture is measured with a humidity meter and an electronic data logger.

Step 2

CM – Calcium Carbide Method

Calcium Carbide Method (CM)

It is a method for rapid determination of water content from the gas pressure developed by the reaction of calcium carbide with the free water of the soil. From the calibrated scale of pressure gauge the percentage of water on total mass of wet soil is obtained and the same is converted to water content and dry mass of soil.

  • The tester samples material over the entire cross-section of the screed with a hammer and a chisel.
  • The sample is placed into a pressure cylinder equipped with a manometer. There, the sample is mixed with calcium carbide.
  • The mixture produces acetylene gas and results in a pressure increase indicated by the manometer.
  • With the help of a conversion table, the moisture content of the screed can be determined.

Step 3

CRH – Corresponding relative humidity Method

Corresponding Relative Humidity Method

The CRH meter works in two modes: In the moisture measuring mode it uses the principle of electrical impedance measurement to give accurate non-destructive moisture readings. In the hygrometer mode it uses a solid state capacitive sensor RH probe to give accurate and fast relative humidity and temperature readings.

CRH method in moisture measuring mode.

The instrument is pressed onto the surface. Electrodes with special spring mounted probes transmit a low frequency signal into the test material. This process of testing can be used to give a clear picture of when and where to place the method in hygrometer mode.

CRH method in hygrometer mode.

Test hole need to be drilled into the substrate. A sleeve with a special probe is inserted into that and covered with the cap. After 3 h the interface is plugged into the probe and the results can be read on the display. The method measures the moisture content in concrete, cement screeds, calcium sulphate screeds and gypsum.

Step 4

substrate drying is necessary if it is too moist

Comparison of both Methods values

Comparison of CM and CRH values.

Screeds are ready to receive flooring if the values below are not exceed. If the substrate is too moist, drying is required as a preparatory measure.

For cement screeds: maximum 2% CM and below 75% CRH.
For calcium sulphate screeds: maximum 3% CM and below 90% CRH.
For heated calcium sulphate screeds: maximum 4% CM and below 95% CRH.

Tips & Tricks

For professionals

How to tackle residual moisture in substrate

In case of moist substrate special primers have to be applied. In the primer segment, the 2-component Epoxy Safety Primer R 755 is Ceresit’s most powerful weapon against rising moisture. It reliably seals cement screeds with a residual moisture content up to 6 % CM and concrete floors up to 7 % CM moisture. It is also suitable for strengthening difficult substrates.

products used

  • ${/content/heliux/adhesive/ceresit/com/en/products/flooring/product#/ceresit-r-755/SAP_0201TNC015M0/searchResult.productFields.MainImageAltText}
    CERESIT R 755
    Reliable moisture barrier effect: up to 6 CM% on cement screeds, up to 7 CM% on concrete floors. Also suitable for producing an epoxy resin screed. 

You may be interested in ...

  • How to prepare floor substrate for tiling
  • How to install tiles and slabs with epoxy adhesives?
  • A-Type waterproofing and tiling of a ground level terrace
  • How to grout tiles and slabs with epoxy grouts?
  • How to apply tile adhesives properly
  • Tiling with CM 77 on difficult substrates
  • How to make a private bathroom waterproof
    How to make a private bathroom waterproof